Tanespimycin (17-AAG)

17-AAG, an analog of geldanamycin, exhibits greater than 100 times higher binding affinity for Hsp90 derived from HER-2-overexpressing cancer cells (BT474, N87, SKOV3 and SKBR3) or BT474 breast carcinoma cells with IC50 values of 5-6 nM. ^[1] 17-AAG causes the degradation of HER2, HER3, Akt, and both mutant and wild-type androgen receptor (AR), leading to the RB-dependent G1 growth arrest of prostate cancer cells such as LNCaP, LAPC-4, DU-145, and PC-3 with IC50 values of 25-45 nM. ^[2] In addition to inducing apoptosis of Ba/F3 cells transformed with wild-type BCR-ABL with an IC50 of 5.2 μM, 17-AAG has the ability to induce apoptosis of cells transformed with T315I and E255K BCR-ABL mutants with IC50 values of 2.3 μM and 1.0 μM, respectively, by inducing the degradation of both wild-type BCR-ABL protein and mutants. ^[3]

17-AAG displays significantly higher binding affinity for Hsp90 from 3T3-src, B16 or CT26 xenografts in nude mice with IC50 values of 8-35 nM as compared with that from the normal tissues with IC50 values of 200-600 nM. ^[1] Administration of 17-AAG (~50 mg/kg) causes significant decline in AR, HER2, HER3, and Akt expression in a dose-dependent manner with >50% decline at dose of 50 mg/kg, resulting in the dose-dependent inhibition of androgen-dependent (CWR22) and -independent (CWR22R and CWRSA6) prostate cancer xenografts growth by 67%, 80% and 68% at dose of 50 mg/kg, respectively. ^[2]

• Hsp90 binding assays

  Purified native Hsp90 protein or cell lysates from HER-2-overexpressing cancer cells (BT474, N87, SKOV3 and SKBR3) or BT474 breast carcinoma cells in lysis buffer (20 mM HEPES, pH 7.3, 1 mM EDTA, 5 mM MgCl₂, 100 mM KCl) are incubated with various concentrations of 17-AAG for 30 minutes at 4 °C, and then incubated with biotin-GM linked to BioMag streptavidin magnetic beads for 1 hour at 4 °C. Tubes are placed on a magnetic rack, and the unbound supernatant removed. The magnetic beads are washed three times in lysis buffer and heated for 5 minutes at 95 °C in SDS–PAGE sample buffer. Samples are analysed on SDS protein gels, and western blots done using indicated antibodies. Bands in the western blots are quantified using the Bio-rad Fluor-S MultiImager, and the percentage inhibition of binding of Hsp90 to the biotin-GM is calculated. The IC50 reported is the concentration of 17-AAG needed to cause half-maximal inhibition of binding.

• Cell lines

  BT474, SKBR3, N87, SKOV3, MCF7, MDA468, Hs578T, Hs578Bst, A549, HT29, U87, SKMG3, HT1080, RPTEC, NDF, HMVEC, HMEC, HUVEC, and PBMC cells

• Concentrations

  Dissolved in DMSO, final concentrations ~10 μM

• Incubation Time

  5 days

• Method

  Cells are seeded in 96-well plates at 2,000 cells per well in a final culture volume of 100 μL for 24 hours before the addition of increasing concentrations of 17-AAG that is incubated for 5 days. Viable cell number is determined using the Celltiter 96 AQueous Nonradioactive Cell Proliferation Assay. The value of the background absorbance at 490 nm (A₄₉₀) of wells not containing cells is subtracted. Percentage of viable cells = (A₄₉₀ of 17-AAG treated sample/A₄₉₀ untreated cells) × 100. The IC50 is defined as the concentration that gave rise to 50% viable cell number.

• Animal Models

  Male nu/nu athymic mice inoculated s.c. with androgen-dependent CWR22 xenograft, and female nu/nu athymic mice inoculated s.c. with androgen-independent xenografts CWR22R and CWRSA6

• Dosages

  ~50 mg/kg

• Administration

  Injection i.p.